Conceiving detectors and/or cameras, which can monitor the spatial and temporal distribution of powerful CO2 laser beams, is an important issue for industrial CO2 laser applications. Beam monitoring is needed for pulsed laser applications, i.e. with a pulse width ranging from picoseconds to milliseconds, as well as for continuous wave (CW) applications. Beam diagnostics prevent laser based industrial processes of going unexpectedly down for some while. In general the laser beam quality deteriorates in time due to laser-induced damage of optical components in the setup or due to thermally induced misalignments in the set-up.
Two fundamental detection mechanisms can be distinguished. Energy detectors respond to temperature changes generated by the incident IR radiation through changes in material properties. Photon detectors generate free electrical carriers through the interaction of photons and bound electrons. Energy detectors are low cost and typically used in single detector applications. However, the simplicity of fabricating large 2D focal plane arrays in semiconductors has lead to the use of photon detectors in almost all advanced IR detection systems. Examples of photon detectors are the quantum-well and quantum-dot infrared detectors, photoconductive and photovoltaic detectors. Energy detectors contain two elements, an absorber and a thermal transducer. Examples are pyroelectric detectors, ferro-electric detectors and thermistors and bolometers. Pyroelectric and ferroelectric detectors comprise a polarized material which when subjected to changes in temperature changes its polarization. In thermistors the resistance of the elements varies with temperature. An example of a thermistor is a bolometer.
In continuous wave (CW) regime, problems arise already with existing commercial detectors, such as photo-electromagnetic (PEM), pyroelectric detectors (PE), for beam powers larger than about 30 W. The sensitivity of existing CW detectors (PE, PEM), . . . can be quite high at low optical powers, i.e. several thousands of VAN, but at higher optical inputs two phenomena yield a strong degradation of the detectors output. At first there is tremendous non-linearity in the electrical output when the optical input increases, the sensitivity can degrade with several orders of magnitude when the input also changes with some orders of magnitude, hence these detectors have a very limited dynamic range. Secondly, beyond some optical power level thermal damage occurs due to their limited thermal evacuation and due to the very local absorption process. In pulsed applications the optical power density can be very high and commercial detectors completely fail to detect these optical power levels. At high optical power levels, i.e. larger than 1 MWcm−2, there is no viable available detector/camera as the existing commercial devices loose their sensitivity at high optical power levels by several orders of magnitude in pulsed applications.
In general it can be said that most of the aforementioned detectors are used to collect the energy emitted by all kind of heated objects. The detected energy is translated into imagery showing the energy difference between objects, thus allowing an otherwise obscured scene to be visualised. Typical applications of these cameras are for thermal imaging, night vision camera, automotive, fire fighting, electronics thermal inspection, and industrial process monitoring.
Consequently, there is a need for the development of novel detector types for the market of laser beam profiling, withstanding much higher power levels. Several systems, used for positioning or measuring power output of lasers are already known.
U.S. Pat. No. 3,624,542 describes a method for checking the luminous power output of a laser comprising a beam splitter. A portion of the laser beam is directed towards a temperature sensitive device. The device is made of a heat-receiving cone which collects a fraction of the beam and directs it to a thermocouple setup. A variation in the laser output leads to a variation in electrical output, which allows continuously monitoring and providing feedback for the output of a laser. The document nevertheless only describes a method for controlling the power output of a light beam, it does not allow to control a light beam profile as it does not provide spatial information. No extension towards array configuration is possible.
Patent application US 2001/0042831 A1 describes a photon detector whereby thin films are deposited on a substrate and whereby the absorption, detection and removal of the generated photon heat is performed in thin films. Nevertheless for real high power systems, the possibility that thermal and mechanical damage occurs is significantly large. In a similar way, patent application US 2003/0164450 A1 describes a method and system for detecting thermal radiation, whereby thin film detector elements are used. Furthermore, US 2003/0164450 A1 describes the use of focussing elements which also is not advantageous if high power laser systems are used.
In patent application U.S. Pat. No. 4,243,888 describes a system for laser beam alignment having a detector disc which is disposed in the laser beam path. The incident laser beam creates thermoelectric effects in the detector which yields voltage signals induced by pulsed heat diffusion. The voltage signals are collected in a short period and integrated to determine the position of the beam and to correct this position if necessary. U.S. Pat. No. 4,243,888 does not provide a device or method for studying the profile of the laser beam light.
The problems with the prior art methods and systems is that they do not allow detection in real high power laser systems as they incorporate thin film detectors or detection systems positioned directly in the laser beam path. Furthermore, most of the systems have the disadvantage that they only allow to monitor the output power of the laser or provide a means for aligning the laser beam.